The use of nucleoside analogues is, and will continue to be for the foreseeable future, the principal means of combating the human immunodeficiency virus (HIV), the etiological cause of the acquired immunodeficiency syndrome (AIDS). The proven efficacy of nucleoside analogues and the lack of currently available alternative compounds underscore the clinical need to mitigate the severe cumulative toxicities associated with their use. It is of paramount clinical concern and the long-term objective of this proposal to improve the use of the anti-HIV nucleoside analogues by preventing the associated toxicities that seriously curtail their use. The most salient toxicities of the nucleoside analogue that preclude their successful long-term administration are: AZT induced anemia, neutropenia and myopathy, ddI and ddC induced pancreatitis, and ddI, ddC and D4T associated peripheral neuropathy. The underlying molecular basis for these toxicities remains to be fully determined. Present evidence, however, would indicate that intracellular accumulation of the nucleoside analogues, their incorporation into cellular DNA and the disruption of mitochondrial DNA by these compounds all contribute to their associated morbidity. With regard to the bone marrow suppressive effect of AZT and given the prohibitive nature of serial bone biopsies, by employing 3H-AZT, we propose to study the metabolism of this compound by examining in vitro the formation and retention of AZTTP in acid soluble form and AZTMP in the DNA of PHA-stimulated peripheral blood mononuclear cells (PBMC). This may provide a preclinical determination of potential AZT marrow toxicity. The disruption of mitochondrial DNA by successive nucleoside analogues is proposed by us to be the underlying cause for the neuropathy and myopathy associated with their use. We propose to use serial determinations of PBMC mitochondrial DNA (mtDNA) content, by sequence specific mtDNA probing, as a surrogate marker for the development of delayed-type toxicity in patients treated with nucleoside analogues. The information should not only provide insight into the host factors determining the effectiveness of the nucleoside analogues as antiviral compounds, but in their toxic proclivities as well. The information gleaned would have clear cut and immediate clinical relevance in deciding the least toxic anti-HIV therapy for a given patient.